1. Field of the Invention
The present invention relates to an exhaust gas recirculation control system for a compression-ignition internal combustion engine or Diesel engine.
An exhaust gas recirculation system for reintroducing part of exhaust gas into intake air to suppress a combustion of fuel for reducing NOx exhausted from an engine has been well known.
In recirculating the exhause gas with a spark-ignition engine having an intake throttle valve, the exhaust gas can be recirculated into the intake air to a desired extent by utilizing pressure differences between the exhaust gas and intake negative pressure downstream of the throttle valve. With a compression-ignition or Diesel enging having no intake throttle valve, however, pressure differences between the exhaust gas and intake negative pressure within a low region of the engine are too little to obtain a determined quantity of exhaust gas recirculation, notwithstanding a great quantity of exhaust gas recirculation is required within the low load region of the engine.
It has been proposed to provide in an intake passage a throttle valve responsive to engine loads to communicate an exhaust gas recirculation with the downstream of the throttle valve in order to control the quantity of the exhaust gas recirculation depending upon the intake negative pressure.
According to this conventional control system, the quantity of exhaust gas recirculation decreases inversely proportionally to the increase of the load of the engine as shown in FIG. 1. It is therefore theoretically successful in decreasing NOx, keeping the high engine performance and preventing "smoke" which is a black exhaust gas containing unburned gases.
In embodying this control system, however, an interlocking mechanism for the throttle valve and a fuel injection control lever unavoidably includes therein plays or clearances which make difficult the proper control of the exhaust gas recirculation flow in an extremely low load region when the throttle valve is almost closed, and a slight deviation of the exhaust gas recirculation flow remarkably increases the smoke in the high load region near the "smoke" limit which means a limit that the exhaust gas containing unburned gases begins to appear black. Accordingly, as the dispersion in dimensional tolerance of parts becomes very important, it is almost impossible to embody the above described exhaust gas recirculation system of the prior art without paying a particular precaution for a quality control of the parts.